Ceramics and process for producing

ABSTRACT

The present invention provides a process for producing alumina matrix carbide and boride reinforced ceramic composites wherein for any particular composite, the relative density is about 97% or more of the theoretical density. The composites are prepared in a container wherein the interior surfaces of the container are graphite and have a protective coating consisting of a first layer comprising silicon carbide and boron carbide with a binder and a second layer comprising silicon carbide particles, wherein the protective coating prevents carbon bleed-through and the protective coating maintains a boride-containing equilibrium atmosphere during the process. The present invention further provides an alumina-based ceramic composite which comprises a metal carbide preferably selected from the group consisting of silicon carbide, titanium carbide and zirconium carbide, and mixtures thereof, and a boride preferably selected from the group consisting of boron carbide, titanium boride, or zirconium boride, and mixtures thereof. Finally, the present invention provides a protective coating for a surface comprising a first layer of a silicon carbide and a boron carbide in a binder and a second layer comprising a silicon carbide wherein the protective coating is able to withstand repeated exposure to high temperature.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] None.

BACKGROUND OF THE INVENTION

[0003] (1) Field of the Invention

[0004] The present invention relates to a process for producing aluminamatrix carbide and boride reinforced ceramic composites wherein for anyparticular composite, the relative density is 97% or more of thetheoretical density. The composites are prepared in a container whereinthe interior surfaces of the container are graphite and have aprotective coating consisting of a first layer comprising siliconcarbide and boron carbide with a binder and a second layer comprisingsilicon carbide particles, wherein the protective coating preventscarbon bleed-through and provides a boride-containing equilibriumatmosphere during the process. The present invention further relates toan alumina-based ceramic composite which comprises a metal carbidepreferably selected from the group consisting of silicon carbide,titanium carbide and zirconium carbide, and mixtures thereof, and aboride preferably selected from the group consisting of boron carbide,titanium boride, or zirconium boride, and mixtures thereof. Finally, thepresent invention relates to a protective coating for an articlecomprising a first layer of a silicon carbide and a boron carbide in abinder and a second layer comprising a silicon carbide particles whereinthe protective coating is able to withstand repeated exposure to hightemperature.

[0005] (2) Description of Related Art

[0006] Aluminum oxide (alumina) based ceramics which contain at least50% alumina and carbides produced by hot pressing or hot isostaticpressing have high strength and excellent resistance to corrosion,oxidation, and wear. However, these alumina-based ceramics have poorstrength and toughness when compared to silicon nitride-based ceramicmaterials. Of particular importance is that alumina-based ceramicsgenerally have relatively poor strength and toughness and are sensitiveto thermal crack formation because the aluminum oxide has relativelypoor thermal conductivity. In the case of metal cutting tools, thisleads to very short tool lives in machining steel, particularly underconditions with short operating times and varying cutting depths.Various attempts have been made to improve the strength, toughness, andthermal conductivity of alumina-based ceramics. To some extent, thethermal properties of alumina-based ceramics has been improved byaddition of titanium carbide or titanium nitride to improve the thermalconductivity of the ceramic. The carbide/nitride also had the effect ofincreasing the hardness of the ceramic. However, the toughness of theceramic material was insufficient for fabricating tools to use forcutting steel. When zirconium oxide was added to the aluminum oxide acomposite was produced which had increased strength and toughness butwith thermal properties not much better than those of pure aluminumoxide. The addition of silicon carbide to aluminum oxide has resulted inan alumina-based ceramic containing silicon carbide whiskers which hasincreased strength and toughness when compared to pure alumina-basedceramics.

[0007] Alumina-based ceramics have been described in the prior art. Forexample, U.S. Pat. No. 4,732,878 discloses an oxidation resistantalumina-based ceramic comprising alumina-silica or alumina-boria-silicaas a first phase and an in situ generated discontinuous carbon secondphase. U.S. Pat. No. 5,418,197 discloses fabrication of an alumina-basedceramic containing homogeneously dispersed silicon carbide whiskers.U.S. Pat. No. 5,538,926 discloses fabrication of alumina-based ceramiccontaining silicon carbide whiskers and one or more oxides of Mg, Si,Ca, Ti, Zr, C, Ni, Y, and rare earth elements. U.S. Re. 32,843 and Re.34,446 disclose fabrication of alumina-based ceramics comprising siliconcarbide whiskers using a hot-press method.

[0008] However, despite the research into developing high densityalumina-based ceramics, there still remains a need for a method thatenables the efficient production of alumina-based ceramics with adensity greater than 97% of the theoretical density for the ceramic andwhich has improved thermal conductivity, strength, and toughness, andincreased resistance to wear, corrosion, and oxidation.

SUMMARY OF THE INVENTION

[0009] The present invention provides a process for producing aluminamatrix carbide and boride reinforced ceramic composites wherein for anyparticular composite, the relative density is 97% or more of thetheoretical density. The composites are preferably prepared in acontainer wherein the interior surfaces of the container are graphiteand have a protective coating consisting of a first layer comprisingsilicon carbide and boron carbide with a binder and a second layercomprising silicon carbide particles, wherein the protective coatingprevents carbon bleed-through and provides boride. During firing, theprotective coating provides boride in an equilibrium atmosphere in thecontainer wherein the boride is at a concentration that inhibitsleeching of the boron or boron carbide from the green preform. Theequilibrium atmosphere also prevents carbon from outside the containerfrom entering the container and impregnating the green preform. Thus,the equilibrium atmosphere enables alumina-based ceramics to befabricated to higher densities. The present invention further providesan alumina-based ceramic composite which comprises a metal carbidepreferably selected from the group consisting of silicon carbide,titanium carbide and zirconium carbide, and mixtures thereof, and aboride preferably selected from the group consisting of boron carbide,titanium boride, or zirconium boride, and mixtures thereof. Finally, thepresent invention provides a protective coating for an articlecomprising a first layer of a silicon carbide and a boron carbide in abinder and a second layer comprising silicon carbide particles whereinthe protective coating is able to withstand repeated exposure to hightemperature.

[0010] Thus, the present invention provides a process for preparation ofa dense alumina-based ceramic composition which comprises: (a) providinga container with a removable closure, wherein inside surfaces of thecontainer and closure are graphite and have been first coated with amixture of metal carbide particles, boride particles, and an organicbinder in water to form a first layer which is then coated with a secondlayer of silicon carbide particles to form a coating which is thendried; (b) introducing into the container a dried green preform madefrom a mixture of an alumina and a metal carbide powder and a boridepowder, wherein the mixture has been milled together; (c) firing thepreform at a temperature sufficient to produce the ceramic compositionwhich has a density of at least 97 percent of a theoretical density forthe ceramic composition.

[0011] In particular, the present invention provides a process forpreparation of a dense alumina-based ceramic composition whichcomprises: (a) providing a container with a removable closure, whereininside surfaces of the container and closure are graphite and have beenfirst coated with a mixture of silicon carbide powder, boron carbidepowder, and an organic binder in water to form a first layer which isthen coated with a second layer of silicon carbide particles to form acoating which is then dried; (b) introducing into the container a driedgreen preform made from a mixture of an alumina and a metal carbidepowder and a boride powder, wherein the mixture has been milledtogether; (c) firing the preform at a temperature sufficient to producethe ceramic composition which has a density of at least 97 percent of atheoretical density for the ceramic composition.

[0012] In a preferred embodiment, the present invention provides aprocess wherein the metal carbide is selected from the group consistingof silicon carbide, titanium carbide, zirconium carbide, and mixturesthereof and the boride is selected from the group consisting of boroncarbide, titanium boride, zirconium boride, and mixtures thereof.Optionally, the present invention further provides a process whereincomposition comprising the preform green includes a sintering aidselected from the group consisting of yttria, rare earths, magnesia,calcia, lanthanides, and mixtures thereof. Preferably, the sintering aidis yttria (Y₂O₃).

[0013] The present invention further provides a process for preparationof a dense alumina-based ceramic composition which comprises: (a)providing a container with a removable closure, wherein inside surfacesof the container and closure are graphite and have been first coatedwith a mixture of silicon carbide powder, boron carbide powder, and anorganic binder in water to form a first layer which is then coated witha second layer of silicon carbide particles to form a coating which isthen dried; (b) introducing into the container a dried green preformmade from a mixture of an aluminum oxide, a silicon carbide powder, anda boron carbide powder, wherein the mixture has been milled together;(c) firing the preform at a temperature sufficient to produce theceramic composition which has a density of at least 97 percent of atheoretical density for the ceramic composition.

[0014] In particular embodiments of the present invention the firstlayer comprises 98 wt % of the silicon carbide powder, 1 wt % of theboron carbide powder, and 2 wt % of the organic binder and the siliconcarbide particles are of about 70 to 120 mesh.

[0015] In the process of the present invention the firing is performedby ramping the temperature to about 500° C. at a rate of between about1° to 5° C. per minute, then to about 1250° C. to 1600° C. at a rate of1° to 10° C. per minute, then to a temperature of between about 1600° to1900° C. at a rate of about 1° to 20° C. per minute, and thenmaintaining the preform at the temperature of 1600° to 1900° C. for atime sufficient to achieve a minimum density of 97%, preferably a timebetween about 10 to 120 minutes. Alternatively, the firing is performedby ramping the temperature to about 500° C. at a rate of between about1° to 5° C. per minute, then to about 1600° C. at a rate of 5° to 20° C.per minute, and then maintaining the preform at the temperature of 1600°to 1900° C. for a time sufficient to achieve a minimum density of 97%,preferably a time between about 10 to 120 minutes. In a preferredembodiment of the present invention, the firing is performed in anatmosphere consisting of an inert gas wherein the gas is selected fromthe group consisting of argon, helium, nitrogen, and mixtures thereof.

[0016] In the process of the present invention, the green preformcomprises 65 to 85 wt % of the alumina with a median particle size inthe size range of 0.4 to 1.5 d₅₀ μm, 0.5 to 20 wt % of the boride with amedian particle size of not more than 30 d₅₀ μm, and 2 to 21.4 wt % ofthe metal carbide with median particle size in the size range of 2 to 10d₅₀ μm. Therefore, the present invention provides a preform greencomposition for producing a ceramic comprising 65 to 85 wt % of thealumina with a median particle size in the size range of 0.4 to 1.5 d₅₀μm, 0.5 to 20 wt % of the boride with a median particle size of not morethan 30 d₅₀ μm, and 2 to 21.4 wt % of the metal carbide with medianparticle size in the size range of 2 to 10 d₅₀ μm. Thus, the presentinvention further provides a fired ceramic composite comprising 65 to 85wt % of the alumina with a median particle size in the size range of 0.4to 1.5 d₅₀ μm, 0.5 to 20 wt % of the boride with a median particle sizeof not more than 30 d₅₀ μm, and 2 to 21.4 wt % of the metal carbide withmedian particle size in the size range of 2 to 10 d₅₀ μm.

[0017] In particular, the present invention provides the composition andthe fired ceramic composite wherein the alumina is 80 wt % with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, the boride isboron carbide, e.g., B₄C, and is 5.4 wt % with a median particle size inthe size range of 3 to 11 d₅₀ μm, and the metal carbide is a mixture ofsilicon carbide which is 2 wt % with a median particle size in the sizerange of 2 to 10 d₅₀ μm and 12.6 wt % with a median particle size in thesize range of 2 to 10 d₅₀ μμm; or wherein the alumina is 80 wt % with amedian particle size in the size range of 0.4 to 1.5 d₅₀ μm, the borideis boron carbide, e.g., B₄C, and is 5.4 wt % with a median particle sizein the size range of 3 to 11 d₅₀ μm, and the metal carbide is siliconcarbide and is 14.6 wt % with a median particle size in the size rangeof 0.5 to 1.0 d₅₀ μm; or wherein the alumina is 78.1 wt % with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, the boride isboron carbide, e.g., B₄C, and is 0.5 wt % with a median particle sizenot more than 12 d₅₀ μm, and the metal carbide is silicon carbide and is21.4 wt % with a median particle size in the size range of 0.5 to 1.0d₅₀ μm; or wherein the alumina is 60 to 85 wt % with a median particlesize in the size range of 0.4 to 1.5 d₅₀ μm, the boride is boroncarbide, e.g., B₄C, and is 1 to 20 wt % with a median particle size inthe size range of 1 to 30 d₅₀ μm, and the metal carbide is siliconcarbide and is 5 to 15 wt % with a median particle size in the sizerange of 0.5 to 20 d₅₀ μm; or wherein the alumina is 60 to 85 wt % witha median particle size in the size range of 0.4 to 1.5 d₅₀ μm, theboride is titanium boride, e.g., TiB₂, and is 1 to 20 wt % with a medianparticle size in the size range of 1 to 30 d₅₀ μm, and the siliconcarbide is 5 to 15 wt % with a median particle size in the size range of0.5 to 20 d₅₀ μm. The carbide and/or boride particles can be a powder orwhiskers. The green preform is made into a shape by slip casting, pillpressing, cold isostatic pressing, extrusion forming, injection molding,or dry bag pressing.

[0018] Further still, the present invention provides a protectivecoating for a surface that is exposed to high temperatures wherein thesurface is graphite prepared by a process comprising providing a mixtureof metal carbide particles, boride particles, and an organic binder inwater to form a first layer on the surface which is then coated with asecond layer comprising metal carbide particles which is then dried toform the protective coating. Preferably, a protective coating for asurface that is exposed to high temperatures wherein the surface isgraphite prepared by a process comprising providing a mixture of siliconcarbide powder, boron carbide powder, and an organic binder in water toform a first layer on the surface which is then coated with a secondlayer comprising metal carbide particles which is then dried to form theprotective coating. In particular, the protective coating wherein thefirst layer comprises 98 wt % of the silicon carbide powder, 1 wt % ofthe boron carbide powder, and 2 wt % of the organic binder and whereinthe second layer of silicon carbide particles is 70 to 120 mesh.

[0019] The present invention further provides a container with aremovable closure for firing ceramics wherein inside surfaces of thecontainer and closure are graphite and have been first coated with amixture of metal carbide particles, boride particles, and an organicbinder in water to form a first layer which is then coated with a secondlayer of metal carbide particles to form a coating which is then dried.Preferably, a container with a removable closure for firing ceramicswherein inside surfaces of the container and closure are graphite andhave been first coated with a mixture of silicon carbide powder, boroncarbide powder, and an organic binder in water to form a first layerwhich is then coated with a second layer of silicon carbide particles toform a coating which is then dried. In particular, a container whereinthe first layer comprises 98 wt % of the silicon carbide powder, 1 wt %of the boron carbide powder, and 2 wt % of the organic binder andwherein the second layer of silicon carbide particles is 70 to 120 mesh.

[0020] Further still, the present invention provides an industrial blastnozzle assembly comprising: a ceramic composite liner having a boreextending therethrough to provide an inlet opening and an outlet openingwherein the ceramic composite liner which comprises 65 to 85 wt % of analumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metalcarbide has a density of at least 97% of a theoretical density for theceramic; and a metal casing having a bore extending therethrough,wherein the liner is mounted in the bore of the metal casing. In apreferred embodiment, the metal casing is a metal selected from thegroup consisting of brass and aluminum. In a particular embodiment ofthe nozzle assembly, the metal casing has a threaded end and the lineris mounted in the bore of the metal casing such that the threaded end ofthe metal casing and the inlet end of the liner form an end which issubstantially flush. optionally, the nozzle assembly further comprises aprotective coating which binds together the liner and metal casing.Preferably, the protective coating is polyurethane. In a preferredembodiment of the liner in the nozzle assembly, the inlet opening has awider diameter than the outlet opening and there is a venturi shape inthe bore between the inlet and the outlet openings. In particularembodiments of the liner for the nozzle assembly, the alumina has amedian particle size in the size range of 0.4 to 1.5 d₅₀ μm, the boridehas a median particle size of not more than 30 d₅₀ μm, and the metalcarbide has a median particle size in the size range of 2 to 10 d₅₀ μm.In a preferred embodiment of the liner for the nozzle assembly, themetal carbide is selected from the group consisting of silicon carbide,titanium carbide, zirconium carbide, and mixtures thereof and the borideis selected from the group consisting of boron carbide, titanium boride,zirconium boride, and mixtures thereof.

[0021] Further still, the present invention provides a liner for anindustrial blast nozzle assembly comprising a ceramic composite having abore extending therethrough to provide an inlet opening and an outletopening wherein the ceramic composite which comprises 65 to 85 wt % ofan alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of a metalcarbide has a density of at least 97% of a theoretical density for theceramic. In a preferred embodiment of the liner, the inlet opening has awider diameter than the outlet opening and there is a venturi shape inthe bore between the inlet and the outlet openings. In particular, thepresent invention provides a liner wherein the alumina has a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, the boride has amedian particle size of not more than 30 d₅₀ μm, and the metal carbidehas a median particle size in the size range of 2 to 10 d₅₀ μm.Preferably, the metal carbide is selected from the group consisting ofsilicon carbide, titanium carbide, zirconium carbide, and mixturesthereof and the boride is selected from the group consisting of boroncarbide, titanium boride, zirconium boride, and mixtures thereof.

[0022] In particular embodiments of the liner for the industrial blastnozzle, the alumina is 80 wt % with a median particle size in the sizerange of 0.4 to 1.5 d₅₀ μm, the boride is boron carbide, e.g., B₄C, andis 5.4 wt % with a median particle size in the size range of 3 to 11 d₅₀μm, and the metal carbide is a mixture of silicon carbide which is 2 wt% with a median particle size in the size range of 2 to 10 d₅₀ μm and12.6 wt % with a median particle size in the size range of 2 to 10 d₅₀μm; or wherein the alumina is 80 wt % with a median particle size in thesize range of 0.4 to 1.5 d₅₀ μm, the boride is boron carbide, e.g., B₄C,and is 5.4 wt % with a median particle size in the size range of 3 to 11d₅₀ μm, and the metal carbide is silicon carbide and is 14.6 wt % with amedian particle size in the size range of 0.5 to 1.0 d₅₀ μm; or whereinthe alumina is 78.1 wt % with a median particle size in the size rangeof 0.4 to 1.5 d₅₀ μm, the boride is boron carbide, e.g., B₄C, and is 0.5wt % with a median particle size not more than 12 d₅₀ μm, and the metalcarbide is silicon carbide and is 21.4 wt % with a median particle sizein the size range of 0.5 to 1.0 d₅₀ μm; or wherein the alumina is 60 to85 wt % with a median particle size in the size range of 0.4 to 1.5 d₅₀μm, the boride is boron carbide, e.g., B₄C, and is 1 to 20 wt % with amedian particle size in the size range of 1 to 30 d₅₀ μm, and the metalcarbide is silicon carbide and is 5 to 15 wt % with a median particlesize in the size range of 0.5 to 20 d₅₀ μm; or wherein the alumina is 60to 85 wt % with a median particle size in the size range of 0.4 to 1.5d₅₀ μm, the boride is titanium boride, e.g., TiB₂, and is 1 to 20 wt %with a median particle size in the size range of 1 to 30 d₅₀ μm, and thesilicon carbide is 5 to 15 wt % with a median particle size in the sizerange of 0.5 to 20 d₅₀ μm.

[0023] Therefore, it is an object of the present invention to provide aprocess for producing an alumina matrix carbide and boride reinforcedceramic composites which have a relative density greater than 97% of thetheoretical density.

[0024] It is a further object of the present invention to provide aprotective coating for an article which protects the article fromrepeated exposure to a carbon atmosphere.

[0025] Further still, it is an object of the present invention toprovide an industrial blast nozzle assembly wherein the nozzle assemblyhas ceramic liner comprising an alumina-based composite containingcarbides and borides which has a relative density greater than 97% ofthe theoretical density.

[0026] These and other objects of the present invention will becomeincreasing apparent with reference to the following description ofpreferred embodiments and examples.

DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a plan view of an industrial blast nozzle assembly 10.

[0028]FIG. 2 is plan view of a ceramic liner 20 for an industrial blastnozzle assembly 10.

[0029]FIG. 3 is a cross-sectional view along the longitudinal axis ofthe ceramic liner 20 for an industrial blast nozzle assembly 10.

[0030]FIG. 4 is a cross-sectional view along the longitudinal axis of anindustrial blast nozzle assembly.

[0031]FIG. 5 is a cross-sectional along line 5-5 of FIG. 4.

[0032]FIG. 6 is a an exploded and cut-a-way view of a container 40 andclosure 42 for fabricating ceramic composites with a protective layer 44on the inner surfaces of the container 40. Inside the container 40 is aboron carbide plate 46 with green preforms 48 of the ceramic liners 20for nozzle assembly 10 placed thereon.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] As used herein, the term “d₅₀” means the median diameter of theparticles. As used herein, the term “mesh” refers to the size ofparticles that are U.S. mesh.

[0034] The present invention provides as process for sintering densealuminum oxide (alumina) based ceramic composites which contain carbidesand borides and has after firing a relative density of about 97% orgreater of the theoretical density for the ceramic. Producing ceramiccomposites with relative densities that approach the theoretical densityfor the composite is an important objective because for every 1%increment of density above a theoretical density of 97% can dramaticallyincrease the structural properties of the fired composite. For example,the present invention provides alumina-based composites and a processfor producing alumina-based ceramics which have a theoretical density ofabout 97% or greater and which have one or more of the followingproperties: increased hardness, toughness, strength, and thermalconductivity which improves resistance to thermal crack formation, andenhanced resistance to oxidation, corrosion, and wear.

[0035] In particular, the present invention provides a process forpreparing multiphase alumina-based ceramics and compositions forpreparing multiphase alumina-based ceramics which comprise a continuousalumina matrix phase and one or more discontinuous phases consisting ofsilicon carbide and boride carbide. The discontinuous phase or phasescan optionally contain various amounts of carbon. Optional still, thealumina phase can contain silica from an amorphous silica or anorganosilane, preferably a trialkoxy organo-functional silane ortetraalkoxysilane and/or boria, preferably boric acid.

[0036] Ceramics, in particular alumina-based ceramics, are generallymanufactured by first forming a green body or shape from a powderfollowed by drying and firing or sintering at a high temperature. Thegreen body forming processes includes, but is not limited to, by slipcasting, pill pressing, cold isostatic pressing, extrusion forming,injection molding, or dry bag pressing. Dry pressing is when the powderis formed into a shape by a mold. Slip casting, is when a slurry ofpowder in water is placed in a porous mold and the water is removed byvacuum or by capillary action. Extrusion forming is when the powder ismixed with water and a plasticizer and is extruded as a thick paste.Injection molding is when the powder is mixed with an organic polymer,heating the mixture to melt the polymer, and injecting the mixture intoa cold mold wherein the polymer solidifies. Other methods for formingpreform greens are cold isostatic pressing and dry bag pressing. Afterforming the green body, the green body is fired or sintered at a hightemperature which sinters the constituents of the ceramic powders toform the ceramic product. The preform green can further include asintering aid, which includes but is not limited to, yttria, magnesia,calcia, lanthanides, and rare earth metals. Preferably the sintering aidis yttria, e.g., Y₂O₃. The sintering aid, which is used to help increasethe density of the ceramic composite, is preferably used in an amount upto and including 5 wt % of the composition.

[0037] As used in the process of the present invention, the preferredceramic powders comprising the composite include alumina, metal carbide,and boride. The metal carbides can be any combination of carbides,preferably a carbide selected from the group consisting of siliconcarbide, titanium carbide, zirconium carbide, and mixtures thereof. Theborides can be any combination of borides, preferably a boride selectedfrom the group consisting of boron carbide, e.g., B₄C, titanium boride,e.g.,TiB₂, zirconium boride and mixtures thereof. The composition of thecomposite can include from about 50 vol % to 95 vol % of alumina andfrom about 0.1 vol % to 50 vol % of any combination of carbides andborides. The alumina is preferably alumina oxide. The particle size ofthe alumina, carbide, and boride can be from about 50 nm to 100 μm. Thehigh density materials produced by the process of the present inventionhave one or more of the following desirable properties: excellentthermal shock resistance, oxidation resistance and high fracturetoughness of silicon carbide, with the hardness, wear resistance and lowspecific gravity of boron carbide. While the carbide and boride can be apowder, the carbide and/or boride can also be provided as particulatematter such as in the form of whiskers.

[0038] The ceramic powders can further contain an organic binder tofacilitate fabrication of preformed shapes with the ceramic powders. Thebinder is about 1 to 5 wt % of the total ceramic powders and isuniformly mixed with the ceramic powders. The binder is temporary and isburned off during the firing or sintering step. The binder is preferablypolyvinyl alcohol or PVP, however, other temporary binders can be used,such as coal tar pitch, long chain fatty material (for example“CARBOWAX” wax), metallic stearates such as aluminum stearates and zincstearates, sugars, starches, alginates, and polymethyl phenylene. Theceramic powders containing the binder are made into preforms by by slipcasting, pill pressing, cold isostatic pressing, extrusion forming,injection molding, or dry bag pressing. During firing, the debinderingor burning out of the binder is conducted under an inert gas, or vacuum,which decomposes the binder so only a small amount of free carbon isleft after the burnout. This also helps the sintering to achieve ahigher density. The debindering can be performed outside the sinteringfurnace as well as inside the sintering furnace. Preferably, thedebindering is performed outside the sintering furnace up to about 600°C.

[0039] In the firing or sintering process, the preformed shapes (greens)are placed in a container such as a crucible or workbox. While thecontainer can be made of a variety of materials which can withstand hightemperatures, a preferred material is graphite. In general, it ispreferable that the environment surrounding the greens in the containercontains silicon carbide and boron carbide phases at a temperaturebetween about 1000° to 2000° C. ; the phases inhibit boride or boroncarbide from leeching from the preform greens. Thus, an importantelement in the process of the present invention is providing a meanswhich can maintain an atmosphere consisting of silicon carbide and/orboron carbide phases. Thus, the present invention further provides inthe process a container wherein the interior surfaces of the containerform a chamber which are coated with an aqueous slurry comprisingsilicon carbide powder and boron carbide powder to form a first layerwhich is then coated with silicon carbide particles to form a secondlayer. The silicon carbide particles are preferably finely divided toform particles with the consistency of a sand or powder. The coatedsurfaces which comprise the protective coating are allowed to dry beforethe greens are placed into the chamber of the container for sintering orfiring. Preferably, the slurry is about 0.1 to 98 wt % of siliconcarbide powder and boron carbide powder. In a most preferred embodiment,the silicon carbide is about 98 wt % and the boron carbide is about 1 wt% and optionally, an organic binder which is about 2 wt % of the slurry.Alternatively, the interior of the container is coated with sinteredsilicon carbide using boron or boron carbide between about 0.1 to 10 wt% as the sintering aid, or sintering parts covered with a siliconcarbide and boron carbide mixture, wherein the silicon and boroncarbides in the mixture are between about 0.1 to 30 wt %. It ispreferable that the second layer comprise silicon carbide particles of amesh between about 70 and 120 mesh. Thus, the protective coatingprovides a means for providing an equilibrium atmosphere that containsboride or boron carbide which inhibits the boride or boron carbide fromleeching from the preform greens.

[0040] The above coating during the firing or sintering process form astrong heat resistant layer or protective coating over the graphitewhich is highly resistant to cracking. The enhanced resistance tocracking prevents bleed-through of the carbon from the graphite into thegreen preform shape during the firing process. Carbon bleed-through isundesirable because the carbon that bleeds through impregnates the greenpreforms within the crucible and lowers the density of the alumina-basedceramic which in turn can adversely affect one or more of the followingproperties of the alumina-based ceramic: strength, hardness, toughness,and thermal conductivity, and resistance to corrosion, oxidation, andwear. The protective coating has a useful life of at least 10 firingsbefore the repeated exposure to the extremely high temperatures duringthe firing or sintering process begins to introduce cracks into theprotective coating which can allow carbon from the graphite to bleedinto the container. The cracks can be repaired by applying a mixture ofthe silicon carbide and boron carbide to the crack. During firing, theprotective coating also provides an equilibrium atmosphere containingboride or boron carbide which inhibits boride or boron carbide fromleeching from the preform greens. Therefore, for fabricatingalumina-based ceramics of high density and with the enhanced propertiesof the ceramics produced according to the process of the presentinvention, it is preferred that the process be performed in a containerwhich has the above protective coating.

[0041] In the process of the present invention, the firing or sinteringis performed in a furnace such as a graphite-element furnace which hasbeen fitted to provide a flow of inert gas throughout the furnace duringthe firing process. In a preferred embodiment, the gas is selected fromthe group consisting of nitrogen, argon, helium, and mixtures thereof.Preferably, the gas is argon. Alternatively, the firing can be performedin a vacuum. The firing is preferably performed wherein the greenpreforms, which optionally have been debindered, are placed in acontainer such as a crucible which has a closure.

[0042] While the container has a closure, the container is not sealedbut the cover does sit in a close spacial relationship to the containerto effectively close the container. Therefore, at high temperatures acarbon atmosphere produced from the furnace, including the heatingelements, carbon refractory, and graphite from the outer surfaces of thecontainer, which are not coated with the protective coating, can bleedcarbon into the container through the gap between the container and theclosure. The carbon in the carbon atmosphere can then impregnate theceramic composite which can change the composition of the ceramic fromits intended composition. Thus, a tight fit between the container andthe closure can achieve a reduction in the amount of carbon that canenter the container. Therefore, the fit between the container and theclosure is important for reducing carbon bleed-through. However, it wasfurther discovered that by maintaining an equilibrium atmosphere insidethe container, the carbon atmosphere produced outside the containerduring firing does not bleed into the container. Furthermore, it wasalso discovered that the combination of protective coating and theequilibrium atmosphere inhibited loss of boride or boron carbide fromthe preform green during the firing process. Boride is volatile and athigh temperatures is leeched from the green preform. The leeching ofboride changes the boride composition of the ceramic from that which wasintended. However, in the process of the present invention, during thefiring process, boride is released from the protective coating andmaintained in the equilibrium atmosphere at a concentration whichmaintains a balance between free boride and boride in the preform green.Therefore, the equilibrium atmosphere enables the boride composition tobe preserved in the preform green during the firing process. Thus, theequilibrium atmosphere enables fabrication of alumina-based ceramicswith densities that approach their theoretical density.

[0043] In the process of the present invention, the firing procedure canuse any of the following sintering schedules or variations thereof.According to a first sintering schedule, the temperature is ramped fromroom temperature to 500±100° C. at a rate of about 1° to 5° C. perminute. The temperature is dependent on the size of the green sample andthe binder. At 500±100° C. the temperature is increased to 1250° C. at arate between 1° to 5° C. per minute and then to a temperature between160° to 1900° C. at a rate between about 1° to 20° C. per minute. Thetemperature is held at the highest temperature for a time sufficient toachieve a density greater than 97%, preferably a time between about 10to 60 minutes with the flow of argon gas at atmospheric pressure. Thetemperature is then returned to room temperature. Thus, the combinationof temperature, gas, container-closure fit, and container coatingprovides an equilibrium atmosphere which prevents introduction of carboninto the ceramic and inhibits loss of boride from the ceramic. Thecombination enables production of high density alumina-based ceramicswhich have a density that is 97% or more of the theoretical densitypossible for the particular composite.

[0044] According to a second sintering schedule, the preformed specimenis placed in the crucible in the furnace with a flow of inert gas. Thetemperature is raised from room temperature to 450±100° C. over a timeperiod of greater than two hours, depending on the size and binder type.Then the temperature is raised to 1300±200° C. within a time framebetween about 1 to 4 hours. Next, the temperature is raised to 1825±50°C. within a time frame of 1 to 4 hours. The temperature is held at1825±50° C. for 10 to 120 minutes before cooling down to roomtemperature.

[0045] According to a third sintering schedule,. the increase intemperature from 500° C. to 1600° C. is performed at a rate of 15° to20° C. per minute. This fast burn or fast firing rate reduces theability of the aluminum to sinter the carbide which typically occurs ifthe temperature passes through 1400° C. at a lower rate. The temperatureis held between 1600° and 1900° C. for 10 to 120 minutes before coolingdown to room temperature. From 1600° to 1900° C. the product densifies.While the preferred sintering schedule is similar to the above firstsintering schedule, the fast burn rate was found to enhance theproduction of high density alumina-based ceramics.

[0046] The present invention also provides particular composites orcompositions which can be used to produce unique alumina-based ceramicproducts which have a density that is preferably about 97% or greater ofthe theoretical density for the ceramic produced therefrom. The presentinvention further provides that the preform green comprising theparticular composite or composition can be fired according to theprocess of the present invention. The particular composites orcompositions which are preferred and which preferably are used as thepreformed green in the process of the present invention have thefollowing formulation.

[0047] Formula 1 consists of 2 wt % of silicon carbide with a medianparticle size in the size range of 2 to 10 d₅₀ μm, 12.6 wt % of siliconcarbide with a median particle size in the size range of 0.5 to 1.0 d₅₀μm, 5.4 wt % of boron carbide with a median particle size in the sizerange of 3 to 11 d₅₀ μm, and 80 wt % of alumina with a median particlesize in the size range of 0.4 to 1.5 d₅₀ μm. Formula 1, which produces aceramic composite with a Rockwell Hardness A of about 92-93 HRA and anIndentation Toughness (KIC) of 5.9 MPa-m^(½), has applications forproducing abrasive blast nozzles (e.g., sand blast nozzles), cuttingtools, and wear resistant parts.

[0048] Formula 2 consists of 14.6 wt % of silicon carbide with a medianparticle size in the size range of 0.5 to 1.0 d₅₀ μm, 5.4 wt % of boroncarbide with a median particle size in the size range of 3 to 11 d₅₀ μm,and 80 wt % of alumina with a median particle size in the size range of0.4 to 1.5 d₅₀ μm. Formula 2 has applications for producing abrasiveblast nozzles (e.g., sand blast nozzles), cutting tools, and wearresistant parts.

[0049] Formula 3 consists of 21.4 wt % of silicon carbide with a medianparticle size in the size range of 0.5 to 1.0 d₅₀ μm, 0.5 wt % of boroncarbide with a median particle size in the size range of less than 12d₅₀ μm, and 78.1 wt % of alumina with a median particle size in the sizerange of 0.4 to 1.5 d₅₀ μm. Formula 3 has applications for producingwear resistant parts, pump seals, and high temperature structuralapplications.

[0050] Formula 4 consists of 21.4 wt % of silicon carbide with a medianparticle size in the size range of 20 to 40 d₅₀ μm, 0.5 wt % of boroncarbide with a median particle size in the size range of less than 12d₅₀ μm, and 78.1 wt % of alumina with a median particle size in the sizerange of 0.4 to 1.5 d₅₀ μm. Formula 4 has applications for hightemperature uses such as crucibles and coatings for crucibles useful formaking ceramics according to the process of the present invention,tubes, etc.

[0051] Formula 5 consists of 5 to 15 wt % of silicon carbide with amedian particle size in the size range of 0.5 to 20 d₅₀ μm, 1 to 20 wt %of boron carbide or titanium boride with a median particle size in thesize range of 1 to 30 d₅₀ μm, and 60 to 85 wt % of alumina with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm. Formula 5 haspotential for a wide range of applications.

[0052] The above composites produce alumina-based ceramics which have adensity that is about 97% or greater of the theoretical density for thecomposite and which have one or more desirable properties such asincreased strength, toughness, hardness, thermal conductivity whichimproves resistance to thermal crack formation, and enhanced resistanceto corrosion, oxidation, and wear. Therefore, the present inventioncomprises high-density alumina-based ceramics comprising any one of theabove compositions. The present invention further comprises high-densityalumina-based ceramics consisting of any one of the above compositeformulas when produced according to the process of the presentinvention.

[0053] The present invention further comprises an industrial blastnozzle assembly. FIG. 1 shows a particular embodiment of an industrialblast nozzle assembly 10 which comprises a ceramic liner 20, a casing30, preferably made from a metal such as brass or aluminum, andoptionally, a protective coating 40, preferably polyurethane, whichinsulates liner 20 from external shock and holds ceramic liner 20 andcasing 30 together.

[0054] The ceramic liner 20 shown in FIG. 2 is cylindrical with a bore21 extending therethrough to provide an inlet opening 22 and an outletopening 23. The bore forms a central venturi nozzle wherein inlet end 22of bore 21 has a wider diameter than outlet end 23. FIG. 3 is across-sectional view along longitudinal axis of ceramic liner 20 showingceramic composite 24, bore 21, inlet 22, and outlet 23 wherein bore 21forms a venturi shape. The ceramic composite which comprises liner 20 is65 to 85 wt % of the alumina, 0.5 to 20 wt % of the boride, and 2 to21.4 wt % of the metal carbide and has a density of at least 97% of thetheoretical density for the ceramic composite. Preferably, the aluminahas a median particle size in the size range of 0.4 to 1.5 d₅₀ μm, theboride has a median particle size of not more than 30 d₅₀ μm, and themetal carbide has median particle size in the size range of 2 to 10 d₅₀μm. In particular embodiments of ceramic liner 20, the ceramic compositecomprises the formulae disclosed herein.

[0055]FIG. 4 shows a cross-sectional view along the longitudinal axis ofnozzle assembly 10 wherein inlet end 22 of liner 20 is inserted throughand mounted in the casing 30 which in this particular embodiment has athreaded end 31 with threads 32 such that the liner ceramic composite 24at inlet end 22 of liner 20 and threaded end 31 of casing 30 form an endwhich is substantially flush. FIG. 4, also shows that in this particularembodiment, the liner 20 and casing 30 are held together with protectivecoating 40. FIG. 5 is a cross-sectional along line 5-5 of nozzleassembly 10 in FIG. 4 showing bore 21 through liner 20, metal casing 30,and protective coating 40. While FIGS. 1 through 4 show an industrialblast nozzle assembly having a casing with a threaded end and aprotective coating, in particular embodiments of the industrial blastnozzle assembly, the casing does not have a threaded end or a protectivecoating or both.

[0056] The ceramic liner 20 for nozzle assembly 10 is fabricatedaccording to the process of the present invention which uses a containerwith the protective coating which enables maintenance of the equilibriumatmosphere. FIG. 6 shows an exploded and cut-a-way view of container 40and closure 42, which is used for sintering preform greens 48 of ceramicliners 20, with protective layer 44 on the inner surfaces of container40. Inside container 40 is a boron carbide plate 46 with green preforms48 of ceramic liners 20 for an industrial blast nozzle assembly placedthereon.

[0057] The following examples are intended to promote a furtherunderstanding of the present invention.

EXAMPLE 1

[0058] This example provides an example of the method of the presentinvention for preparing high density alumina-based ceramics. Inparticular, a ceramic from a composite consisting of 78.2 wt % ofAluminum oxide (Al₂O₃), 21.3 wt % of silicon carbide, and 0.5 wt % ofboron carbide (B₄C).

[0059] A graphite crucible with a tight lid was used. The interiorsurface of this crucible and lid were coated with a liquid coating layerwhich was immediately covered with a layer of 70 to 120 mesh siliconcarbide sand which acts as a sealer. The coating layer was applied as aslurry comprising 98 wt % silicon carbide powder, 1 wt % boron carbideand 2 wt % of an organic binder in water of an amount to provide aslurry of sufficient fluidity to evenly coat the interior surfaces ofthe crucible and lid. After covering the coating layer with the siliconcarbide sand, the slurry was allowed to dry. The specially coatedcrucible was then used for sintering the composite prepared below.

[0060] The composite was formed as follows. A mixture was made whichcontained 156 gram of aluminum oxide powder with a median particle sizein the size range of 0.4 to 1.5 d₅₀ μm, 42.48 gram of silicon carbidepowder with a median particle size in the size range of 0.5 to 1.0 d₅₀μm, 1.021 gram of boron carbide with a median particle size less than 12d₅₀ μm, and 6 gram of PVP were dispersed in 100 gram of water. Themixture was ball milled for 24 hours to produce a slurry. The solidgreen samples were prepared by either slip casting of the above slurry,or compacting the spray dried powder from the mixture into the solidgreen samples (dry pressing). After allowing the solid green samples todry, the dried green samples were placed inside the above crucible withthe special coating and the crucible was covered with the lid. The greencomposite was calculated to comprise 78.2 wt % of Aluminum oxide, 21.3wt % of silicon carbide, and 0.5 wt % of boron carbide.

[0061] The sintering schedule was as follows. The crucible containingthe solid green samples was placed in a graphite-element furnace andheated at a ramp from 2° to 5° C. per minute up to 500° C., then from 5°to 10° C. per minute up to 1250° C., and finally from 2° to 20° C. perminute to 1600° to 1900° C. The temperature was held within the finalrange for 10 to 60 minutes under a flow of argon gas at atmosphericpressure. Inside the crucible an atmosphere was created which was thespecific boride containing environment for sintering the composite.

[0062] Afterwards, the density of the fired samples was measured byArchimedes' method as a percentage of the theoretical density (T.D.).The results are shown in Table 1. TABLE 1 Sample Forming SinteringHolding Time Density # Method Temp. (° C.) (min.) (% T.D.) 1 SlipCasting 1825 ± 25 20 99.1 2 Slip Casting 1825 ± 25 25 98.6 3 DryPressing, 1825 ± 25 20 98.5 1 ton/cm²

[0063] This example demonstrates that the process of the presentinvention produces high density ceramics with a composite density above98% of the theoretical density. This is extremely important because each1% increment of density above 97% can dramatically increase theproperties of the composite. Because the protective coating on theinterior surfaces of the crucible prevented carbon bleed-through, thisexample demonstrates that the protective coating on the interiorsurfaces of the carbon/graphite crucible provides a practical and lowcost means for production of alumina-based ceramics of very highdensity. The equilibrium atmosphere, which was used in the process forsintering composites, is an important condition of the process whichprevents carbon from entering the crucible and impregnating the ceramicand simultaneously providing boride from the protective coating whichinhibits the leeching of boride from the ceramic. Thus, the process ofthe present invention enables production of multiphase ceramiccomposites of high density.

EXAMPLE 2

[0064] This example demonstrates the process of the present inventionusing a composite that consisted of 80 wt % alumina, 14.6 wt % siliconcarbide, and 5.4 wt % boron carbide.

[0065] Samples were prepared according to the process in Example 1except that composite comprised 80 wt % alumina with a median particlesize in the size range of 0.4 to 1.5 d₅₀ μm, 14.6 wt % silicon carbidewith a median particle size in the size range of 0.5 to 1.0 d₅₀ μm, and5.4 wt % boron carbide with a median particle size in the size range of3 to 11 d₅₀ μm. The composite was ball milled for 24 hours as in Example1 and the green samples were prepared as in Example 1. The sinteringschedule was as in Example 1. The density of the fired samples wasmeasured by Archimedes' method as a percentage of the theoreticaldensity (T.D.) as shown in Table 2. TABLE 2 Sample Forming SinteringHolding Time Density # Method Temp. (° C.) (min.) (% T.D.) 4 Slipcasting 1825 ± 25 20 99.1 5 Slip casting 1825 ± 25 25 98.4 6 DryPressing, 1825 ± 25 24 98.4 1 ton/cm² 7 Dry Pressing, 1825 ± 25 24 98.61 ton/cm² 8 Dry Pressing, 1850 ± 25 30 98.6 1 ton/cm²

[0066] This example further demonstrates that using a second composite,the process of the present invention was still able to produce a highdensity ceramic with a density above 98% of the theoretical density. Theexample further shows that the second composite, like the composite inexample 1 can be used to produce ceramics with desirable properties.

EXAMPLE 3

[0067] This example provides the formulae for several ceramic compositesof the present invention for producing alumina-based high densityceramics.

[0068] Formula 1 consists of 2 wt % of silicon carbide with a medianparticle size in the size range of 2 to 10 d₅₀ μm, 12.6 wt % of siliconcarbide with a median particle size in the size range of 0.5 to 1.0 d₅₀μm, 5.4 wt % of boron carbide with a median particle size in the sizerange of 3 to 11 d₅₀ μm, and 80 wt % of alumina with a median particlesize in the size range of 0.4 to 1.5 d₅₀ μm. Formula 1 was used toproduce a high density ceramic using a sintering schedule similar tothat in Example 1.

[0069] Formula 2 consists of 14.6 wt % of silicon carbide with a medianparticle size in the size range of 0.5 to 1.0 d₅₀ μm, 5.4 wt % of boroncarbide with a median particle size in the size range of 3 to 11 d₅₀ μm,and 80 wt % of alumina a median particle size in the size range of 0.4to 1.5 d₅₀ μm. Formula 2 was used to produce a high density ceramicusing a sintering schedule similar to that in Example 1.

[0070] Formula 3 consists of 21.4 wt % of silicon carbide with a medianparticle size in the size range of 0.5 to 1.0 d₅₀ μm, 0.5 wt % of boroncarbide with a median particle size less than 12 d₅₀ μm, and 78.1 wt %of alumina with a median particle size in the size range of 0.4 to 1.5d₅₀ μm. Formula 3 was used to produce a high density ceramic using asintering schedule similar to that in Example 1.

[0071] Formula 4 consists of 21.4 wt % of silicon carbide with a medianparticle size in the size range of 20 to 40 d₅₀ μm, 0.5 wt % of boroncarbide with a median particle size less than 12 d₅₀ μm, and 78.1 wt %of alumina with a median particle size in the size range of 0.4 to 1.5d₅₀ μm. Formula 4 was used to produce a high density ceramic using asintering schedule similar to that in Example 1.

[0072] Formula 5 consists of 5 to 15 wt % of silicon carbide with amedian particle size in the size range of 0.5 to 20 d₅₀ μm, 1 to 20 wt %of boron carbide or titanium boride with a median particle size in thesize range of 1 to 30 d₅₀ μm, and 60 to 85 wt % of alumina with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm. Formula 5 was usedto produce a high density ceramic using a sintering schedule similar tothat in Example 1.

EXAMPLE 4

[0073] Table 3 shows a comparison of erosion tests for composites madefrom Formula 1 to published erosion tests for other composites.Fabrication of the ceramic product comprising Formula 1 used the processwhich was shown in Example 1. TABLE 3 Comparison of Erosion Test for(ASTM G76-83) Sample Erosion Rate (cm³/g) at 90° Density (g/cm³) Formula1 1.37 × 10⁴ 3.645 Formula 1 1.09 × 10⁴ 3.638 Formula 1 1.43 × 10⁴ 3.662Silicon nitride¹  2.8 × 10⁴ 3.23 Cemented Co₂  1.4 × 10⁴ 14.846 tungstencarbide¹ ROCTEC 5001  0.1 × 10⁴ 15.55

[0074] Table 3 shows that Formula 1 produces a ceramic with an averageerosion rate of less than silicon nitride-based ceramic composites andless than or equal to the 6% Co cemented ceramic carbides. The advantageof the present invention is that Formula 1 has less than {fraction(1/4)} the weight of tungsten carbide. While the erosion rate forFormula 1 was greater than the ROCTEC 500, it has the advantage ofreduced cost and weight compared to the ROCTEC.

EXAMPLE 5

[0075] This example compares the service life of ceramics made fromcomposite Formula 1 to other ceramics made from other composites in themanufacture of nozzles.

[0076] The process was as in Example 1 except that the production firingprocedure used the following sintering schedule. The temperature wasramped from room temperature to 600° C. at a rate of 2 to 3° C. perminute in a vacuum. Then 600° to 1150° C. at a rate of 5° to 6° C. perminute. Then from 1150° to 1825° C. ±25° C. at a rate of 10° to 20° C.per minute followed by holding at 1825° C. ±25° C. for 20 to 40 minutes.The service life was 50 to 100 hours under production using aluminumoxide grit as the abrasive.

[0077] The ceramic composite products produced herein have been definedin terms of the composition of the starting material comprising thegreen preform. Therefore, the ceramic composites are to be viewed as thekind of ceramic composite that is produced from the particularcompositions disclosed herein. Defining the ceramic composite in termsof the starting material is used as a matter of convenience because itis well known in the manufacture of ceramic composites that thecomponents comprising the starting material are volatile and willvaporize to different extents and/or form alloys with other componentsof the starting material, and/or undergo other transformations duringthe firing process. Thus, after firing, the components in the ceramiccomposite will not necessarily be in the same proportion as they existedin the starting material. Therefore, while the ceramic composites arediscussed in terms of their starting material, it is not to be construedthat the final ceramic product will have the same composition as thestarting material.

[0078] While the present invention is described herein with reference toillustrated embodiments, it should be understood that the invention isnot limited hereto. Those having ordinary skill in the art and access tothe teachings herein will recognize additional modifications andembodiments within the scope thereof. Therefore, the present inventionis limited only by the claims attached herein.

I claim: 1- A process for preparation of a dense alumina-based ceramiccomposition which comprises: (a) providing a container with a removableclosure, wherein inside surfaces of the container and closure aregraphite and have been first coated with a mixture of metal carbideparticles, boride particles, and an organic binder in water to form afirst layer which is then coated with a second layer of silicon carbideparticles to form a coating which is then dried; (b) introducing a driedgreen preform made from a mixture of alumina and a metal carbide powderand a boride powder, wherein the mixture has been milled together, intothe container; (c) firing the preform at a temperature sufficient toproduce the ceramic composition which has a density of at least 97percent of a theoretical density for the ceramic composition. 2- Aprocess for preparation of a dense alumina-based ceramic compositionwhich comprises: (a) providing a container with a removable closure,wherein inside surfaces of the container and closure are graphite andhave been first coated with a mixture of silicon carbide powder, boroncarbide powder, and an organic binder in water to form a first layerwhich is then coated with a second layer of silicon carbide particles toform a coating which is then dried; (b) introducing into the container adried green preform made from a mixture of alumina and metal carbide andboride powders, wherein the mixture has been milled together; (c) firingthe preform at a temperature sufficient to produce the ceramiccomposition which has a density of at least 97 percent of a theoreticaldensity for the ceramic composition. 3- The process of claim 2 whereinthe metal carbide is selected from the group consisting of siliconcarbide, titanium carbide, zirconium carbide, and mixtures thereof andthe boride is selected from. the group consisting of boron carbide,titanium boride, zirconium boride, and mixtures thereof. 4- A processfor preparation of a dense alumina-based ceramic composition whichcomprises: (a) providing a container with a removable closure, whereininside surfaces of the container and closure are graphite and have beenfirst coated with a mixture of silicon carbide powder, boron carbidepowder, and an organic binder in water to form a first layer which isthen coated with a second layer of silicon carbide particles to form acoating which is then dried; (b) introducing into the container a driedgreen preform made from a mixture of aluminum oxide, silicon carbide,and boron carbide powders, wherein the mixture has been milled together;(c) firing the preform at a temperature sufficient to produce theceramic composition which has a density of at least 97 percent of atheoretical density for the ceramic composition. 5- The process of claim2 or 4 wherein the firing is performed by ramping the temperature toabout 500° C. at a rate of between about 1° to 5° C. per minute, then toabout 1250° C. to 160° C. at a rate of 1° to 10° C. per minute, then toa temperature of between about 1600° to 1900° C. at a rate of about 1 to20° C. per minute, and then maintaining the preform at the temperatureof 1600° to 1900° C. for a time sufficient to achieve 97% or more of thetheoretical density. 6- The process of claim 4 wherein the firing isperformed by ramping the temperature to about 500° C. at a rate ofbetween about 1° to 5° C. per minute, then to about 1600° C. at a rateof 5 to 20° C. per minute, and then maintaining the preform at thetemperature of 1600° to 1900° C. for a time sufficient to achieve 97% ormore of the theoretical density. 7- The process of claim 2 wherein thegreen preform comprises 65 to 85 wt % of the alumina with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, 0.5 to 20 wt % ofthe boride with a median particle size not more than 30 d₅₀ μm, and 2 to21.4 wt % of the metal carbide with a median particle size in the sizerange of 2 to 10 d₅₀ μm. 8- The process of claim 7 wherein the greenpreform comprises a sintering aid selected from the group consisting ofyttria, rare earths, lanthanides, magnesia, and calcia. 9- The processof claim 7 wherein either the metal carbide or the boride is provided aswhiskers. 10- The process of claim 7 wherein the alumina is 80 wt % witha median particle size in the size range of 0.4 to 1.5 d₅₀ μm, theboride is boron carbide and is 5.4 wt % with a median particle size inthe size range of 3 to 11 d₅₀ μm, the metal carbide is silicon carbideand is 2 wt % with a median particle size in the size range of 2 to 10d₅₀ μm and 12.6 wt % with a median particle size in the size range of 2to 10 d₅₀ μm. 11- The process of claim 7 wherein the alumina is 80 wt %with a median particle size in the size range of 0.4 to 1.5 d₅₀ μm, theboride is boron carbide and is 5.4 wt % with a median particle size inthe size range of 3 to 11 d₅₀ μm, and the metal carbide is siliconcarbide and is 14.6 wt % with a median particle size in the size rangeof 0.5 to 1.0 d₅₀ μm. 12- The process of claim 7 wherein the alumina is78.1 wt % with a median particle size in the size range of 0.4 to 1.5d₅₀ μm, the boride is boron carbide and is 0.5 wt % with a medianparticle size not more than 1.2 d₅₀ μm, and the metal carbide is siliconcarbide and is 21.4 wt % with a median particle size in the size rangeof 0.5 to 1.0 d₅₀ μm. 13- The process of claim 7 wherein the alumina is60 to 85 wt % with a median particle size in the size range of 0.4 to1.5 d₅₀ μm, the boride is boron carbide and is 1 to 20 wt % with amedian particle size in the size range of 1 to 30 d₅₀ μm, and the metalcarbide is silicon carbide and is 5 to 15 wt % with a median particlesize in the size range of 0.5 to 20 d₅₀ μm. 14- The process of claim 7wherein the alumina is 60 to 85 wt % with a median particle size in thesize range of 0.4 to 1.5 d₅₀ μm, the boride is titanium boride and is 1to 20 wt % with a median particle size in the size range of 1 to 30 d₅₀μm, and the silicon carbide is 5 to 15 wt % with a median particle sizein the size range of 0.5 to 20 d₅₀ μm. 15- The process of claim 2 or 4wherein the first layer comprises 98 wt % of the silicon carbide powder,1 wt % of the boron carbide powder, and 2 wt % of the organic binder.16- The process of claim 2 or 4 wherein the silicon carbide particlesare of about 70 to 120 mesh. 17- The process of claim 1, 2 or 4 whereinthe green preform is made into a shape by pill casting, slip casting,isostatic pressing, or dry bag pressing. 18- The process of claim 1, 2or 3 wherein the firing is performed in an atmosphere consisting of aninert gas wherein the gas is selected from the group consisting ofargon, helium, nitrogen, and mixtures thereof. 19- A green composite forproducing a ceramic comprising 65 to 85 wt % of an alumina with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, 0.5 to 20 wt % ofa boride with a median particle size of not more than 30 d₅₀ μm, and 2to 21.4 wt % of a metal carbide with a median particle size in the sizerange of 2 to 10 d₅₀ μm. 20- The green composite of claim 19 wherein thealumina is 80 wt % with a median particle size in the size range of 0.4to 1.5 d₅₀ μm, the boride is boron carbide and is 5.4 wt % with a medianparticle size in the size range of 3 to 11 d₅₀ μm, the metal carbide issilicon carbide and is 2 wt % with a median particle size in the sizerange of 2 to 10 d₅₀ μm and 12.6 wt % with a median particle size in thesize range of 2 to 10 d₅₀ μm. 21- The green composite of claim 19wherein the alumina is 80 wt % with a median particle size in the sizerange of 0.4 to 1.5 d₅₀ μm, the boride is boron carbide and is 5.4 wt %with a median particle size in the size range of 3 to 11 d₅₀ μm, and themetal carbide is silicon carbide and is 14.6 wt % with a median particlesize in the size range of 0.5 to 1.0 d₅₀ μm. 22- The green composite ofclaim 19 wherein the alumina is 78.1 wt % with a median particle size inthe size range of 0.4 to 1.5 d₅₀ μm, the boride is boron carbide and is0.5 wt % with a median particle size not more than 12 d₅₀ μm, and themetal carbide is silicon carbide and is 21.4 wt % with a median particlesize in the size range of 0.5 to 1.0 d₅₀ μm. 23- The green composite ofclaim 19 wherein the alumina is 60 to 85 wt % with a median particlesize in the size range of 0.4 to 1.5 d₅₀ μm, the boride is boron carbideand is 1 to 20 wt % with a median particle size in the size range of 1to 30 d₅₀ μm, and the metal carbide is silicon carbide and is 5 to 15 wt% with a median particle size in the size range of 0.5 to 20 d₅₀ μm. 24-The green composite of claim 19 wherein the alumina is 60 to 85 wt %with a median particle size-in the size range of 0.4 to 1.5 d₅₀ μm, theboride is titanium boride and is 1 to 20 wt % with a median particlesize in the size range of 1 to 30 d₅₀ μm, and the metal carbide issilicon carbide and is 5 to 15 wt % with a median particle size in thesize range of 0.5 to 20 d₅₀ μm. 25- The green composite of claim 19further comprising a sintering aid selected from the group consisting ofyttria, rare earths, lanthanides, magnesia, and calcia. 26- The processof claim 19 wherein either the metal carbide or the boride is providedas whiskers. 27- A fired ceramic composite comprising 65 to 85 wt % ofan alumina with a median particle size in the size range of 0.4 to 1.5d₅₀ μm, 0.5 to 20 wt % of a boride with a median particle size of notmore than 30 d₅₀ μm, and 2 to 21.4 wt % of a metal carbide with a medianparticle size in the size range of 2 to 10 d₅₀ μm and which has adensity of at least 97% of a theoretical density for the ceramic. 28-The fired ceramic composite of claim 27 wherein the alumina is 80 wt %with a median particle size in the size range of 0.4 to 1.5 d₅₀ μm, theboride is boron carbide and is 5.4 wt % with a median particle size inthe size range of 3 to 11 d₅₀ μm, the metal carbide is silicon carbideand is 2 wt % with a median particle size in the size range of 2 to 10d₅₀ μm and 12.6 wt % with a median particle size in the size range of 2to 10 d₅₀ μm. 29- The fired ceramic composite of claim 27 wherein thealumina is 80 wt % with a median particle size in the size range of 0.4to 1.5 d₅₀ μm, the boride is boron carbide and is 5.4 wt % with a medianparticle size in the size range of 3 to 11 d₅₀ μm, and the metal carbideis silicon carbide and is 14.6 wt % with a median particle size in thesize range of 0.5 to 1.0 d₅₀ μm. 30- The fired ceramic composite ofclaim 27 wherein the alumina is 78.1 wt % with a median particle size inthe size range of 0.4 to 1.5 d₅₀ μm, the boride is boron carbide and is0.5 wt % with a median particle size not more than 12 d₅₀ μm, and themetal carbide is silicon carbide and is 21.4 wt % with a median particlesize in the size range of 0.5 to 1.0 d₅₀ μm. 31- The fired ceramiccomposite of claim 27 wherein the alumina is 60 to 85 wt % with a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, the boride isboron carbide and is 1 to 20 wt % with a median particle size in thesize range of 1 to 30 d₅₀ μm, and the metal carbide is silicon carbideand is 5 to 15 wt % with a median particle size in the size range of 0.5to 20 d₅₀ μm. 32- The fired ceramic composite of claim 27 wherein thealumina is 60 to 85 wt % with a median particle size in the size rangeof 0.4 to 1.5 d₅₀ μm, the boride is titanium boride and is 1 to 20 wt %with a median particle size in the size range of 1 to 30 d₅₀ μm, and themetal carbide is silicon carbide and is 5 to 15 wt % with a medianparticle size in the size range of 0.5 to 20 d₅₀ μm. 33- A protectivecoating for a surface that is exposed to high temperatures wherein thesurface is graphite prepared by a process comprising providing a mixtureof metal carbide particles, boride particles, and an organic binder inwater to form a first layer on the surface which is then coated with asecond layer comprising metal carbide particles which is then dried toform the protective coating. 34- The container of claim 33 wherein themetal carbide is silicon carbide and the boride is boron carbide. 35-The protective coating of claim 34 wherein the first layer comprises 98wt % of the silicon carbide powder, 1 wt % of the boron carbide powder,and 2 wt % of the organic binder. 36- The protective coating of claim 33wherein the second layer is silicon carbide particles of 70 to 120 mesh.37- A container with a removable closure for firing ceramics whereininside surfaces of the container and closure are graphite and have beenfirst coated with a mixture of metal carbide particles, borideparticles, and an organic binder in water to form a first layer which isthen coated with a second layer of metal carbide particles to form acoating which is then dried. 38- The container of claim 37 wherein metalcarbide is silicon carbide and the boride is boron carbide. 39- Thecontainer of claim 38 wherein the first layer comprises 98 wt % of thesilicon carbide powder, 1 wt % of the boron carbide powder, and 2 wt %of the organic binder. 40- The container of claim 37 wherein the secondlayer is silicon carbide particles of 70 to 120 mesh. 41- An industrialblast nozzle assembly comprising: (a) a ceramic composite liner having abore extending therethrough to provide an inlet opening and an outletopening wherein the ceramic composite liner which comprises 65 to 85 wt% of an alumina, 0.5 to 20 wt % of a boride, and 2 to 21.4 wt % of ametal carbide has a density of at least 97% of a theoretical density forthe ceramic; and (b) a metal casing having a bore extendingtherethrough, wherein the liner is mounted in the bore of the metalcasing 42- The nozzle assembly of claim 41 wherein the metal casing is ametal selected from the group consisting of brass and aluminum. 43- Thenozzle assembly of claim 41 wherein the metal casing has a threaded endand the liner is mounted in the bore of the metal casing such that thethreaded end of the metal casing and the inlet end of the liner form anend which is substantially flush. 44- The nozzle assembly of claim 41wherein a protective coating binds together the liner and metal casingto provide the industrial blast nozzle assembly. 45- The nozzle assemblyof claim 44 wherein the protective coating is polyurethane. 46- Thenozzle assembly of claim 41 wherein the inlet opening has a widerdiameter than the outlet opening and there is a venturi shape in thebore between the inlet and the outlet openings. 47- The nozzle assemblyof claim 41 wherein the alumina has a median particle size in the sizerange of 0.4 to 1.5 d₅₀ μm, the boride has a median particle size of notmore than 30 d₅₀ μm, and the metal carbide has a median particle size inthe size range of 2 to 10 d₅₀ μm. 48- The nozzle assembly of claim 41wherein the metal carbide is selected from the group consisting ofsilicon carbide, titanium carbide, zirconium carbide, and mixturesthereof and the boride is selected from the group consisting of boroncarbide, titanium boride, zirconium boride, and mixtures thereof. 49- Aliner for an industrial blast nozzle assembly comprising a ceramiccomposite having a bore extending therethrough to provide an inletopening and an outlet opening wherein the ceramic composite whichcomprises 65 to 85 wt % of an alumina, 0.5 to 20 wt % of a boride, and 2to 21.4 wt % of a metal carbide has a density of at least 97% of atheoretical density for the ceramic. 50- The liner of claim 49 whereinthe inlet opening has a wider diameter than the outlet opening and thereis a venturi shape in the bore between the inlet and the outletopenings. 51- The liner of claim 49 wherein the alumina has a medianparticle size in the size range of 0.4 to 1.5 d₅₀ μm, the boride has amedian particle size of not more than 30 d₅₀ μm, and the metal carbidehas a median particle size in the size range of 2 to 10 d₅₀ μm. 52- Theliner of claim 49 wherein the metal carbide is selected from the groupconsisting of silicon carbide, titanium carbide, zirconium carbide, andmixtures thereof and the boride is selected from the group consisting ofboron carbide, titanium boride, zirconium boride, and mixtures thereof.